For example, when a nut is manufactured, a nut blank formed with a prepared hole for an internal thread is first prepared, and then an internal thread is formed in the inner periphery of the prepared hole. Since such a prepared hole is generally formed by forging or pressing, a sheared surface and a broken surface tends to arise in the inner periphery of the prepared hole, so that the inner periphery of the prepared hole may slightly vary due to the sheared and broken surfaces. Therefore, when an internal thread is formed by a normal cutting tap having no reamer portion, the inner periphery of the internal thread may vary (i.e., the inner peripheries of the crests of the thread ridges on the internal thread may not be uniform) due to sheared and broken surfaces, thereby making it impossible to obtain required accuracy in the internal thread.
In order to overcome such a problem, Japanese Unexamined Utility Model Application Publication No. H01-148229 (hereinafter, JP H01-148229) proposes a cutting tap capable of forming an internal thread with high accuracy even when a prepared hole for the internal thread has an inner periphery of which the dimensional accuracy is low.
The cutting tap of JP 1101-148229 includes a cutting portion, and a complete thread portion continuous with the axial rear side of the cutting portion. The cutting portion comprises a plurality of thread ridges formed such that with the effective diameter of the thread ridges uniform in the axial direction, the major diameter of the thread ridges gradually decreases toward the axial front side of the cutting portion. The complete thread portion comprises a plurality of thread ridges having a complete shape, and formed such that the effective diameter and the major diameter of the thread ridges are made uniform in the axial direction. The thread ridges of the cutting portion and the complete thread portion are arranged at a constant pitch in the axial direction.
The cutting tap of JP 1101-148229 is configured such that the root diameter of roots formed between the respective adjacent pairs of thread ridges of the cutting portion is different from that of roots formed between the respective adjacent pairs of thread ridges of the complete thread portion. Specifically, the root diameter of the complete thread portion is larger than that of the cutting portion. Due to this configuration, after the thread ridges of the cutting portion cut the inner periphery of a prepared hole such that an internal thread is formed in the prepared hole, the roots formed between the respective adjacent pairs of thread ridges of the complete thread portion thinly cut the crests of the thread ridges of the internal thread (cut portion corresponding to the inner peripheral surface of the prepared hole).
The inventors of the present application experimentally prepared a cutting tap as disclosed in JP H01-148229, and performed an experiment to actually form an internal thread by use of this cutting tap. As a result of this experiment, it turned out that torque required for cutting is too large, and thus the cutting tap of JP H01-148229 is not practical.
Generally, respective thread ridges on the outer periphery of a cutting tap have effective diameter reliefs by which the effective diameter of the thread ridges gradually decreases from the front side toward the rear side of each thread ridge in the rotation direction of the cutting tap, the effective diameter reliefs being set at 0 (zero) or a minute value. The reason why the effective diameter reliefs are set at 0 (zero) or a minute value is because if the effective diameter reliefs are set at a large value, the cutting tap will not be able to be guided accurately, so that the thread ridges of the internal thread will be deformed (i.e., expanded).
Thread ridges on the outer periphery of a cutting tap are reflexively formed by removing, from the material of the cutting tap (tap blank), portions corresponding to the thread grooves between the respective adjacent pairs of thread ridges. Roots formed between the respective adjacent pairs of thread ridges on the outer periphery of the cutting tap have root diameter reliefs by which the root diameter of the roots gradually decreases from the front side toward the rear side of each root in the rotation direction of the cutting tap. The root diameter reliefs are set to be equal to the effective diameter reliefs of the thread ridges, namely, set at 0 (zero) or a minute value.
Therefore, as in JP H01-148229, when the inner periphery of the prepared hole for an internal thread is cut by the roots formed between the respective adjacent pairs of thread ridges on the outer periphery of the cutting tap, with no radial gaps or substantially no radial gaps defined between the inner periphery of the prepared hole and the roots between the respective adjacent pairs of thread ridges on the outer periphery of the cutting tap, the cutting tap is rotated relative to the prepared hole, so that its rotational resistance is made large. Specifically, if the cutting tap cuts the inner periphery of the prepared hole with an extremely small cutting stock (e.g., if the cutting stock is 0.05 mm or less in diameter), the cutting tap barely can form an internal thread in the prepared hole. However, if the cutting tap forms an internal thread with a cutting stock having more than 0.05 mm (e.g. 0.1 mm or over), the rotational resistance of the cutting tap is made extremely large.
Therefore, for example, if a step having 0.05 mm or over in diameter is formed on the inner periphery of the prepared hole for an internal thread (specifically, if the inner diameter of the prepares hole varies midway by 0.1 mm or over), the cutting tap of JP H01-1482291 is unable to form an internal thread with high accuracy in such a prepared hole.
It is an object of the present invention to provide a cutting tap capable of forming an internal thread with high accuracy even when the prepared hole for the internal thread has an inner periphery of which the dimensional accuracy is low.